This invention relates to a system for detecting a breakage of a nuclear fuel rod in a nuclear reactor.
When a nuclear fuel rod disposed at the core of a nuclear reactor is damaged, a temperature distribution at the core of the nuclear reactor varies, leading to a failure of the stable operation on one occasion and a complete failure of the operation of the nuclear reactor on the other. The failure of the nuclear fuel rod leads to leakage of fission products which in turn enter into a coolant or its cover gas.
As a system for detecting failure of a nuclear fuel rod in the nuclear reactor is generally used a system for examining whether or not a fission product emitting a delayed neutron into a coolant is dissolved and a system for examining a gaseous fission product is mixed in the cover gas. However, the former system shows a poor detection sensitivity and when a failure of the nuclear fuel rod is detected there is a fear that a great damage is already been developed and in consequence undesirable result is obtained from the standpoint of securing the safety of the unclear reactor. In the latter system, it is impossible to distinguish whether a damaged portion is located, for example, in a plenum above the nuclear fuel rod or at a fuel-filling portion which exerts a groove influence over the operation of the nuclear reactor. In order to eliminate such an inconvenience the following system has been proposed. That is, the failure of the nuclear fuel rod is detected by examining the concentration of 135.sub.I utilizing the phenomenon that when the fuel-filling portion of the nuclear fuel rod is failed a fission product emitting a delayed neutron leaks from the failed portion of the rod, together with the 135.sub.I, into a coolant.
This conventional detection system will now be explained below by referring to FIG. 1.
Reference numeral 2 shows a nuclear reactor of the type in which a coolant 6 is circulated around a core 4, and 8 a gas-liquid separator in which an inert cover gas 10 is contained. The separator 8 is adapted to temporarily retain a coolant 12 sampled from the nuclear reactor and transfer into a cover gas 10 135m.sub.Xe produced by decay of 135.sub.I which is already contained in the coolant. The separator 8 has a first inlet 14 connected through a pump means 18 and valve 20 over a pipe 16 to the interior of the nuclear reactor and a second inlet 28 connected through a valve over a pipe 30 to a cover gas supply source, not shown. The separator 8 has a first outlet 22 connected through a valve 26 to the interior of the nuclear reactor and a second outlet 34 connected through a pipe 36 over a pipe 36 to a spent gas treating system, not shown. The cover gas 10 prevents oxidation of the coolant 12 introduced into the separator 8. The separator 8 has a third inlet 40 connected through a valve 44 over a pipe 42 to an inert gas supply system, not shown. Between the inlet 28 and the outlet 34 of the separator 8 is provided a cover gas passage, i.e., a pipe 46 over which a valve 48, pump means 50, vapor trapping means 51, radioactivity detecting means 52 and fission products trapping means 54 and valve 56. The valves 20, 26, 32, 38, 44, 46 and 56 as well as the pumps 18 and 50 are of electromagnetic type and are controlled by the outputs of a control means 58.
The above-mentioned detection system is carried out as follows:
Suppose that a cover gas is being introduced into the separator 8. When the valves 20 and 26 are opened to permit the pump means 18 to be operated, the coolant 6 is sampled from the interior of the nuclear reactor 2 and fed into the separator 4. Then, the valves 20 and 26 are closed to stop the operation of the pump 18, stopping the introduction of the coolant into the separator 8. Then, the valve 44 is opened to permit an inert gas to be introduced into the coolant 12 and 135m.sub.Xe produced by iodine 135.sub.I decay before the coolant is sampled from the nuclear reactor is transferred into the cover gas. Then, the valve 44 is closed and valves 32 and 38 are opened to permit a supply of the inert gas into the coolant 12 to be interrupted. At the same time the cover gas is introduced into the spent gas treating system for exhaust and a fresh cover gas is supplied to the separator from the cover gas supply source. Then, the valves 32 and 38 are closed to stop a supply of the cover gas. This state is maintained for a predetermined time period, permitting decay into 135m.sub.Xe of iodine 135.sub.I contained in the coolant. After the passage of a predetermined period the valve 44 is opened to permit an inert gas to be blown into the coolant 12 in the separator 8. Xenon 135m.sub.Xe decayed from iodine 135.sub.I for the above-mentioned predetermined time period is transferred into the cover gas. Then, the valve 44 is closed and valves 48 and 56 are opened to permit the pump means to be operated. The pump means 50 is operated to cause the 135m.sub.Xe -entrained cover gas to be introduced through the vapor trapping means into the radioactivity of 135.sub.Xe is counted. The concentration of 135.sub.I is proportionately measured from the intensity of radioactivity of 135m.sub.Xe counted. Such an operation is repeated for measurement of a concentration variation of 135.sub.I and the failure of the nuclear fuel rod is detected from the concentration variation of 135.sub.I. However, the conventional detection system has the following disadvantages:
(1) Since the detection sensitivity is determined by the amount of a coolant sampled out of the nuclear reactor, i.e., by the volume of the separator 8, a large capacity gas-liquid separator 8 is necessarily required to elevate the detection sensitivity, resulting in a bulky system.
(2) In order to enhance the detection sensitivity a large quantity of coolant is held in the gas-liquid separator and at the same time an inert gas is blown into the separator, thus producing a synergistic action to cause a larger quantity of coolant to be introduced in the vapor form into the radioactivity detecting means. Removal of the gaseous coolant makes the system complicated in construction and bulky in size.